This invention relates to optical spectrum analysis and, more particularly, to measurement of the spectrum of a modulated optical signal. Specifically, the invention is directed to a method and apparatus for measurement of the spectrum of a modulated optical signal using a gated modulation source and a self-homodyne detection method based on the use of an optical delay circuit for recovering the optical field spectrum on a modulated optical signal.
The power spectrum of an optical source determines the performance of any optical device, such as a fiber optic system or associated component, that operates on that source. For example, if a laser transmitter feeds into an optical fiber cable for a link to an optical receiver elsewhere in a fiber optic system, the power spectrum of the laser determines the amount of pulse distortion due to dispersion in the optical fiber and thus the effectiveness of the communication link.
Various techniques for measuring this power spectrum are known. Unfortunately, they all have limitations and/or disadvantages in performing power spectrum measurements.
One known technique involves the use of a grating spectrometer. However, in practice, resolution requirements often exceed those possible with a grating spectrometer.
Other known techniques employ Fabry Perot, Mach Zehnder, or Michelson discriminators. However, the presence of AM confuses measurements performed with these discriminators.
Another known technique utilizes a scanning Fabry Perot spectrometer. However, this spectrometer has limited dynamic frequency range if operated over a wide spectral band.
Finally, a technique for synthetic heterodyne interferometry for semiconductor laser spectral analysis is disclosed in Abitbol, C., Gallion, P., Nakajima, H., and Chabran, C.: "Analyse la Largeur Spectrale d'un Laser Semiconducteur par Interferometrie Heterodyne Synthetique," J. Optics (Paris), 1984, Vol. 15, No. 6, pp. 411-418. The laser is frequency shift keyed by superimposing a small amplitude square-wave signal on a bias injection current. The optical field is analyzed by an unbalanced Mach Zehnder single-mode fiber interferometer which includes an optical delay circuit. A detector at the output of the interferometer acts as an optical product detector. Unfortunately, the modulation is constrained to be a square wave, and the modulation rate is tied to the delay in the optical circuit, so that the square wave has a period of twice the delay.